Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which is commonly referred to as “logging”, can be performed using several methods.
In conventional oil well wireline logging, a probe, which is housing formation sensors, is lowered into the borehole when the well, or parts of the well, has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the probe is attached to a conductive wireline that suspends the probe in the borehole. Power is transmitted to the sensors and instrumentation in the probe through the conductive wireline. Similarly, the instrumentation in the probe communicates information to the surface by means of electrical signals transmitted through the wireline.
An alternative logging method is collecting data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimise performance while minimising downtime.
Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD”. Typically, the downhole sensors employed in MWD applications are positioned in a cylindrical drill collar which is positioned close to the drill bit. The MWD system then employs a system of telemetry in which the data acquired by the sensors is transmitted to a receiver located on the surface.
There are a number of telemetry systems in the prior art which seek to transmit information regarding downhole parameters up to the surface without requiring the use of a wireline. Of these, the mud pulse system is one of the telemetry systems most widely used for MWD applications. The mud pulse system of telemetry creates “acoustic” pressure signals in the drilling fluid which is circulated under pressure through the drill string during drilling operations. The information acquired by the downhole sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The information is received and decoded by a pressure transducer and computer at the surface. One problem with mud pulse systems is the data transfer rate which for increased amounts of information becomes insufficient.
An alternative to the mud pulse system is to use the wireline used to lower a logging tool into the borehole. Data transfer via a wireline has been developed by using discrete multi-tone modulation (DMT). Such a system typically comprises a surface transceiver, a cable and a downhole transceiver coupled to the surface transceiver via the cable. The downhole transceiver communicates to the surface transceiver using DMT modulation to transmit telemetry information over a set of frequency sub channels allocated for uplink communications. The surface transceiver may likewise communicate to the downhole transceiver using DMT modulation to transmit information over a set of frequency sub-channels allocated for downlink communications. The number of uplink and downlink communications sub-channels is preferably variable, and can preferably be changed depending on the operating mode of the system. This allows additional sub-channels to be allocated for downlink communications during programming and configuration of the downhole equipment, and additional sub-channels to be allocated for uplink communications during normal logging operations.
However, downhole operation becomes increasingly demanding in terms of supplied power to a downhole operating system. The use of driving units for being able to move around in the borehole, the use of one or several logging techniques in one downhole operating system and typically the use of several different power tools to alter or maintain the well have increased the need for downhole power significantly. When increasing the power supplied to a wireline, higher data transmission voltages are needed for transmitting data by means of DMT due to an increasing noise level stemming from an increased supply voltage. Merely increasing the data transmission voltage in known systems leads to more dissipated heat in the electronics of the data transmission system, which is a common problem in downhole operation tools, since heat transfer can be practically impossible in a downhole environment. The increased dissipation of heat limits the ability to transfer data at high rates while providing high power for the downhole operation system in known systems.
Information is the key to being profitable in the oil and gas industry. The more information one has regarding location and migration patterns of hydrocarbons within a hydrocarbon reservoir, the more likely it is that that reservoir can be tapped at its optimal location and utilised to its full potential. To this end, new and more sophisticated sensor arrangements are routinely created and placed in wireline probes, so much so that the information carrying capacity of traditional wireline telemetry techniques are becoming inadequate. Also due to the increased need for power in downhole operation systems, it would be desirable to have a communication technique which can support both high speed communications between downhole sensors and a surface installation while providing a high power to the downhole operation system.